Improved means to manipulate fatty acid compositions, from biosynthetic or natural plant sources, are needed. For example, edible oil sources containing the minimum possible amounts of saturated fatty acids are desired for dietary reasons and alternatives to current sources of highly saturated oil products, such as tropical oils, are also needed.
Higher plants appear to synthesize fatty acids via a common metabolic pathway in plant plastid organelles (i.e., chloroplasts, proplastids, or other related organelles) as part of the Fatty Acid Synthase (FAS) complex. Fatty acids are used in plant membranes and in neutral lipids that are formed for energy storage in developing seed tissues, and the like.
Unsaturated fatty acids originate from the desaturation of stearoyl-acyl carrier protein (hereinafter "acyl carrier protein" may also be referred to as "ACP") to form oleoyl-ACP, a monounsaturated fatty acid, in a reaction catalyzed by a soluble plastid delta-9-desaturase, also known as "stearoyl-ACP desaturase". Thus, in the plant FAS pathway, stearoyl-ACP desaturase catalyzes the production of the monounsaturate oleoyl-ACP (C18:1-ACP) from saturated stearoyl-ACP (C18-ACP). The desaturase enzyme functions to add a first double bond in the eighteen member long stearoyl-ACP carbon chain in accordance with the following reaction (I): EQU Stearoyl-ACP+ferredoxin(II)+O.sub.2 +2H.sup.+ .fwdarw.oleoyl-ACP+ferredoxin (III)+2H.sub.2 O.
Unreacted stearoyl-ACP and oleoyl-ACP may be further metabolized to produce stearic and oleolic fatty acid residues. In plant species naturally low in saturated fat content (e.g., safflower, rapeseed), it is noted that greater than 90% of all stearoyl-ACP synthesized is converted on to the oleoyl thioester.
Stearoyl-ACP desaturase has been studied in partially purified preparations from numerous plant species. Reports indicate that the protein is a dimer, perhaps a homodimer, displaying a molecular weight of 68 kD (.+-.8 kD) by gel-filtration and a molecular weight of 36 kD by SDS-polyacrylamide gel electrophoresis.
Relevant Literature
A 200-fold purification of safflower stearoyl-ACP desaturase was reported by McKeon & Stumpf in 1982, following the first publication of their protocol in 1981. McKeon, T. & Stumpf, P. J.Biol.Chem. (1982) 257:12141-12147; McKeon, T. & Stumpf, P. Methods in Enzymol. (1981) 71:275-281.